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aging process

With increasing age, tendons, skin, and even blood vessels lose elasticity. This is due to the formation of cross-links between or within the molecules of collagen (a fibrous protein) that give elasticity to these tissues. The “cross-linking” theory of aging assumes that similar cross-links form in other biologically important molecules, such as enzymes. These cross-links could...

...with age. Insoluble collagen then builds up with age as a result of synthesis exceeding removal, much like another fibrous tissue, the crystalline lens of the eye. With increasing age, the number of
cross-linkages within and between collagen molecules increases, leading to crystallinity and rigidity, which are reflected in a general body stiffness. There is also a decrease in the relative amount...

human

blood vessels

...thickening of the walls of larger blood vessels with an increase in connective tissue. The connective tissue itself becomes stiffer with increasing age. This occurs because of the formation of cross-links both within the molecules of collagen, a primary constituent of connective tissue, and between adjacent collagen fibres. These changes in blood vessels occur even in the absence of the...

lung

The lung also contains elastin and collagen to give it elastic properties. As indicated previously, the formation of cross-links in elastin and collagen that takes place with aging reduces the elastic properties of the lung.

skin

...to the presence of fibres of the proteins elastin and collagen. Studies of the minute structures of the skin show a gradual reduction in elastin. In addition, the collagen fibres show an increase in cross-links, which greatly restricts the elastic properties of the collagen network.

collagen

When collagen is treated with tannic acid or with chromium salts, cross links form between the collagen fibres, and it becomes insoluble; the conversion of hide into leather is based on this tanning process. The tanned material is insoluble in hot water and cannot be converted to gelatin. On exposure to water at 62° to 63° C (144° to 145° F), however, the cross links formed by...

diene polymers

In commercial use, diene polymers are invariably converted to thermosetting elastomeric network polymers by a process called cross-linking or vulcanization. The most common method of cross-linking is by addition of sulfur to the hot polymer, a process discovered by the American Charles Goodyear in 1839. The relatively small number of cross-links imparts elastic properties to the polymer; that...

elastomers

The most important ingredients are those, known as the cure package, that cause interlinking reactions to take place when the mix is “cured.” In order to minimize the risk of premature cure, they are usually added at the end of mixing. The cure package usually consists of sulfur and one or more “accelerators” (e.g., sulfenamides, thiurams, or thiazoles), which make the...

...greatly impeded. The linking process is often called curing or, more commonly, vulcanization (after Vulcan, the Roman god of fire). More accurately, the phenomenon is referred to as cross-linking or interlinking, because this is the essential chemical reaction.

Elastomers are typically amorphous with low cross-link density (although linear polyurethane block copolymers are an important exception). This gives them low to moderate modulus and tensile properties as well as high elasticity. For example, elastomeric devices can be extended by 100 to 1,000 percent of their initial dimensions without causing any permanent deformation to the material....

radiation

...chain molecules that make up the polymer, either directly or through the mediation of imbedded, light-sensitive “activators.” This results in intermolecular bonding, a process called cross-linking. The entire polymeric coating, typically on the order of tenths of millimetres thick (depending on the application), becomes so highly cross-linked as to take on the character of a...

surface coatings

Some of the highest-performance coatings films are based totally on the reacting of polymer precursors to build up a three-dimensionally cross-linked network. This is at once both a very old and a very new technology. During the Middle Ages drying oils were used without solvent to formulate a paint that formed films totally by oxidative cross-linking. Drying oils are natural products such as...

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